Image receiving tube



My 24, 1938. P 'T. FARNswoRTH 2,118,186

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May 24, 19318. P. T. FARNswo'RTH 2,118,186

IMAGE RECEIVING TUBE Filed July` l5, 1935 ,3 Sheets-Sheet 2 fullurlINcANDEscANT LAMP 32 @fao v INsuLAl-oe 72 w BAS: Wm:

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fllllallllllhlli May 24, 1938. P. T. FANswoRTH I 2,118,186

IMAGE RECEIVING TUBE Fi-ledJuly 15, 1955 3 Shee'cs-Sheei;i 3

INVENToRY PH/o 7: FARNSWORTH.

' ATTORNEYS Patented May 24, 1938 'UNITED STATES lPA'rrN'r orrlcl:l I l'nuca morons I Philo T. Farnsworth, ssn signor to Farnsworth SanFrancisco, Calif.,

forni:

Application July l5,

4 Claims.

My invention relates to athermionic device, and more particularly to a.cathode ray tube adapted to produce an electron image of greaterintensity than can be ordinarily produced di- 5 rectly by the impact of4an optical image upon a photoelectric surface.

Y The present application is a continuation in part of my inventiondisclosed and claimed as to system and method ln my application, SerialNo. 29,242 led July 1, i935v for an Electron image amplifier, thisapplication dealing solely with tube structure.

Among the objects `of my invention are: To provide a photoelectric tubehaving a high output; to provide-a photoelectric tube which whenenergized vvillproduce an output current greater than can be produced bythe impact of lightl upon the photoelectrlc surface alone; to provide animage: amplifying tube having an effective photoelectric emitter ofgreater sensitivity than has hitherto been produced; and to provide asimple and efcient electron image amplifying tube particularly suitablefor use in television systems for the creation of 'a train of televisionsignals. n

Other objects oi my invention will be apparent or will be specicallypointed out in the description forming a part oi this specincation, butI do not limitl myself to the embodiment of the'invention hereindescribed, as various iorms may be adopted within the scope of theclaims.

In my prior application above referred to, I have described and claimedan embodiment ci a method of electron image amplification employing thetube of the instant application. in previous applications and patents ofmine I have described television transmitting apparatus and systemswherein an optical image of the object or Y picture held is' thrown upona photosensitive cathode and the emitted electrons are accelerated andfocused to form au electron image. By electron image i mean a planethrough which the electron stream passes, the electron density of whichovaries spatially across the stream in the same manner as theillumination density varies across the optical image. En other words,the electron values represent spatially, the illumination ci the picturefield.

l The electron stream iorming this image may be deected by means weilknown in the art, out preferably by magnetic means to pass over anaperture in .auch e. manner as to eect the scan` ning of the image.Selected portions of the electron stream passing through the apertureare collected to form a picture current or train oi picture signalswhich may be amplified and mod- Francisco, Calif., as- TelevisionIncorporated, a corporation of Cali- 1935, Serial N0.'31,410

(Cl. Z50-27.5)

@ated upon a radio wave, or. transmitted by wire. This method o'ftelevision transmission offers the advantage of having no moving partsand of being suitable for the electrical projection of pictures havingany desired flneness of detail.

The principal weakness of this method lies in the fact'that only arelatively small portion of the electrons emitted from the totalphotoelectr'ic area is used at any given instant and at the present timephotoelectric emission is relatively small 1 in intrinsic value.Therefore, the highest possible sensitivity must be obtained from thepho-- toelectric surfaces and even then high gain ampliliers arenecessary in order that satisfactory picture currents can be obtained.With small output currents, attempts to amplify the signals above acertain level bring in background noise, Shottke eect and otherordinarily negligible factors which tend to make the amplified picturecurrents unsatisfactory and distorted, and the received picture lackingin the detail which it would have if such interference were not present.

The tube oi the present invention enables the fundamental principle ofmy previous inventions to be retained While other desirable features areadded. An electron image corresponding to the optical image is formedand is thereafter dissected as before. In the present tube, however, theimage has a considerably higher average value than the previous devicebecause of the fact that a space charge is formed, the electrons in thespace charge being released by the ac. tion of the light in the opticalimage. I am therefore able to produce electron images with the presentdevice which are far more powerful than the electron images which areheretofore produced. Under these circumstances, when the picture isscanned, picture currents of much greater amplitude are obtained in thetube out put thereby eliminating high gain amplifiers with theirobjectionable features.

Describing my invention in broad terms, l pre ier to utilize an envelopecontaining a source of electrons, these electrons being preferably lowve lccity electrons. in certain cases, Il may use a thermionic emitterior this source but in other cases, i prefer to utilize e. photoelectricsurface and dood the surface with long wavelength light in order toobtain a stream of low velocityv electrons therefrom. I then position inthe path of this stream an epertured electrode or grid capable of havinga. charge ilxed thereon in accordance with the spatial distribution ofillumination intensities existing in an optical image.

At the other end of the tube, I prefer to position an aperturedelectrode, the aperture being of elemental area and positioned to allowelectrons to pass therethrough and I place, immediately be-A hind theaperture, a vcollecting anode to inter-V cept electrons passing throughthe aperture. The tube when energized is supplied with moving magneticfields which scan the electron image past the aperture so thatsuccessive elementary areas thereof may be collected to produce a trainof television signals.

Various other modications and applications of my invention will beapparent to those skilled in the art andfor other broad aspects of myinvention lI prefer to refer to a detailed description of severalpreferred embodiments of my invention as shown in the drawings, of whichFigure 1 is a longitudinal sectional view of a television dissector tubeprovided with focusing and scanning coils, together with means forprojecting an optical image therein.

Figure 2 is a diagrammatic circuit reduced to lowest terms showing howthe device of Figure 1 may be operated to produce a train of televisionsignals.

Figure 3 is a detailed sectional view of one form of grid.

Figure 4 is a detailed view in section of another form of grid.

Figure 5 is a cross sectional view of another form of dissector tubeshowing diiferential illuf. mination of the photosensitive surfacetherein.

Figure 6 is a cross sectional view of a portion of the control grid ofthe tube showing in Figure 5.

Figure 7 is a diagrammatic circuit showing how the tube of Figure 5 maybe operated.

VFigure 8 is a longitudinal sectional view of another embodiment of myinvention.

Figure 9 is a cross sectional view which is identical with crosssectional views of the device shown in Figures 1, 5 and 8, lookingtoward the charge storage electrode.

Figure 10 is a perspective view of a mesh mosaic.

Figure 11 is a diagrammatic view of apparatus used in forming a silvermosaic.

Figure 12 is a longitudinal sectional view of another modification of myinvention.

Figure 13 is a cross sectional view of a composite grid having twophotoelectric surfaces.

Describing the apparatus in detail, without reference to the operationthereof, which will be taken up later, and referring directly to themodiilcation shown in Figure 1, an envelope I is provided at one end andwith a transparent window 2 in front of which is placed an opticalsystem 4 in such a manner as to focus an optical image of an object uponacharge storage electrode or grid 5 positioned adjacent the opposite endof the tube. On the other side of this grid there is positioned a wideangle electron gun comprising a coneshaped apertured anode 6, a controlgrid 1, and preferably an indirectly heated cathode 9. 'I'his assemblyis preferably mounted on a stem I0 in an extension arm II of theenvelope.

Adjacent the window end of the tube an apertured anode assembly isprovidedcomprising a g A -finger tube I2 haying an aperture I 4 facingthe ly back of the aperture I 4,

vides a longitudinal magnetic field between the vgrid 5 and the scanningaperture I4 for the purpose of maintaining the electron image in properspatial relationship during its passage therebe tween.

The tube is also provided with exterior coils 22 and 23 positionedsubstantially at right angles to each other for moving the electronimage in two directions over the scanned aperture I4, the magneticfields of these coils being formed by energization by scanningoscillators 25 and 24 respectively.

Several different types of grid structures may be used with this device,but in any case the grid member 5 is apertured preferably with an areadevoted to the apertures equal to that of the supporting material. Forexample, I may make, in certain cases, `my grid entirely'of insulatingmaterial as shown in Figure 3. Here the insulator 26 is provided withapertures 2l and is also provided with a layer of caesium on silveroxide 29. f This layer is formed in the usual manner for formingphotoelectric surfaces, the silver being deposited thereon in such amanner that it congeals in droplets so that a mosaic is formed with thecaesium deposited thereon, leaving photoelectric islands more or lessuniformly'insulated one from the other, as is well known in the art.

Another form of grid is shown in Figure 4 where the base material 30 isa conducting material such .as nickel wire, for example, which hasdeposited thereon a layer of insulating material 3i and over' theinsulating material the mosaic 29 of caesium on silver oxide is formed,as described before. 'Ihe preferred way of forming the grid of thislatter construction is to utilize a mesh screen of nickel, for example,having rather larger spaces between the grid wires than the area of thewires themselves and smoking the entire screen with burning magnesiumuntil the wires are covered with magnesium oxide to aV point where thespaces are approximately equal to the area of the composite member.Caesium on silver oxide is then formed upon the magnesium base to formthe mosaic, as will be later described.

When I desire to form a mosaic on an insulating surface of a mesh gridfabric, I have found that it is-not necessary to follow the complicatedand uncertain process above referred to.

In Figure 11 I have shown a preferred apparatus for forming a mosaic onthe mesh grid. The grid is covered with insulating material, at least onthe side facing the optical image, preferably by smoking with magnesiumoxide as above described, and silver is evaporated thereon preferablyfrom substantially a. point source |00, a convenient means being theapplication of eddy currents from a coil IOI energized by an oscillator|02 to a container |03 having an aperture |04 facing the grid andenclosing silver metal |05.

'I'he silver, being evaporated n vacuo, travels in straight lines andtherefore casts sharp shadows. As the individual wires inthe Vmeshalternately pass over and under the wires at right angles to them, thesilver deposit is separated into small rectangles the size of the mesh.No silver is deposited under the overhang of the wires, and not only doI obtain substantially -perfect insulation between the silver islands,but I am able to yaccurately control the size of the islands by changingthe mesh of the'fabric. A mosaic of this type is shown in Figure 10 inperspective and. 1

in Figure 13 in cross section.

when the tube is formed this im is completely oxidized, utilizing, as iscustomary, a radio-frequency discharge in pure oxygen.

I also prefer to completely clean up the Vexcess caesium after the tubeis formed. There are a number of ways of accomplishing such a cleanupbut I prefer to include within the tube, either connected to the cathodeor the anode, a fairly large surface of pure silver which may beoxidized with a fairly heavy coating. After the tube is formed,therefore, this surface is capable of absorbing a large amount ofcaesiurn,A even after the thin film of silver on` the grid has taken upits maximum.

The preferred procedure, therefore, after the surfaces it is desired tosensitize have reached maximum sensitivity, is to lower the temperaturesomewhat and bake the tubeout -in this lower temperature for asufiiciently long time until the excess of 'caesium within the tubeisall taken upv by the accessory oxidized silver surface.

Another modification of my invention is shown in Figure 5 and in thiscase a photoelectric emitter has been substituted for the electron gun,the photoelectric emitter in this case comprising preferably acontinuous photoelectric surface 32 formed on a base member 34,preferably a silver plate. 'Ihis plate is supported on the stem I0 inany customary manner. The grid 5 is then positioned immediately in frontof.) and parallel to the photoelectric surface, and in this case thegrid 5 comprises preferably a conductor having an insulating surface.While the entire --grid structure may be an insulator, I prefer toutilize the conducting base 30 provided with a relatively thin layer ofmagnesium oxide formed thereon as above described, and in this case I d0not place upon the insulator any photoelectric material.

Referring directly to the modification shown in Figure 1 connected as inFigure 2 and assuming that it has a grid as shown in Figure 3; in otherwords, a grid which is completely an insulator,

`the anode 6 of the electron-gun is connected to the' cathode in serieswith an anode battery 35 and the cathode 9 is energized in the usualmannerso that the entire grid structure 5 will be bombarded with asuitable amount of 100 to 300 volt electrons. The grid will thenassume anegative charge just suiilcient to prevent these electrons from strikingit, thus forming a space charge immediately back of the grid. Thepotential of the grid, that is, the normal uniform potential of thegrid, will be .largely determined by a small number of electrons havinglthe highest velocity and due to leakage in the grid structure, theentire grid will assume a uniform charge equilibrium.

inasmuch as the grid in this instance is entirely formed of insulatingmaterial, the uniform grid charge in the structure shown in Figure 3will be slightly higher than in the structure shown in Figure 4, wherethe insulating layer is formed on a base wire 30, as I prefer to connectthis base wire to the anode 6 by aconnection 38. In this case, thevnegative charge, because of greater leakage opportunity will beslightly' less than when the entire grid is formed of insulatingmaterial.

After the grid has assumed a uniform negative charge and the spacecharge has been formed y behind the grid due to thisnegative charge, an

optical image of'an object 31 is focused by means of optical system 4 onthe side of grid 5 opposite to that being bombarded. This optical imagefalls on the mosaic photoelectric layer 29 and causes emissiontherefrom,l the electrons being drawn toward anode I2, those of someparticular elementary area entering aperture I4 and being collected oncollecting plate I6. In order to create electron traversal of the tube,anode I2 is maintained at a positive potential by means of an anodebattery 39 and the collecting plate.Y I6 is maintained at a potentialpositive to anode I2 by collecting source 40. 'I'he difference betweenthe number of electrons collected by the collecting plate I6 and fingerI2 passing through output resistor I4 I creates a potential availablefor further use in output leads 42.

When electrons are emitted vfrom the mosaic surface due to the action ofthe image light, they will of course be emitted in proportion to theintensity of the light falling on the individual mosaic islands and theislands will become more or less positive due to the loss of electronsin l Adifferent portions of the grid to a new point of equilibrium 'andresults in the formation of a charge image on the grid representing inintensity the light intensity of the image. Electrons from the spacecharge developed back of the grid 5 are thus able to be drawn throughthe grid toward theV anode I2 and due to the fact that the number drawnthrough at any particular elemental area is controlled by the charges onelemental areas of the grid, there will be formed in space between thegrid and the anode, an electron image. This electron image is maintainedin spatial relationship by the focusing coil I9 and is scanned acrossaperture I4 by the moving magnetic fields developed by the scanningcoils and generators positioned as previously described.

If desired, lthe output of theelectron gun can be modulated at radiofrequencies by means of the gun grid 1 supplied with a radio frequencymodulation voltage through input lead 44 and blocking condenser 45. Thisallows output amplication with this type of 'amplifier and in far lessamplification and with consequent quietness and freedom frominterference and distortion.

Another apparatus for producing electron storage for the purposeofprovidingv an amplified electron image is shown inv-Figure 5vandconnectedas shown in Figure 8. In this case, I do not use an electrongun for the source of eleotrons forming a uniform electron stream, but Iprefer to use a nat photoelectric cathode 32.l

This cathode is preferably formed as is customary in the art on a silverplate 34 by oxidation and the deposition of caesium until maximumsensitivity to light is obtained and is preferably not a mosaic. 'Ihecathode is then flooded with infra-red and red light from lamps 46 untilthe cathode develops in a specific instance for example, 100microamperes current. Grid 5 in this case is not photoelectric, but ispreferably composed of a base wire 30 of nickel having an insulatingcoating thereon preferably of magnesium oxide formed as above described.

,In operation, the object 31 is illuminated solely by sunlight, forexample, or by incandescent or arc lamps l1, care being taken that noneof the white light reaches cathode 32 except that which is reflectedfrom the object 31 and focused on the cathode I2 by lens I. This isaccomplished in practice by the use oi. reflectors 43 on the whitelights and as it is also desirable that no red light illuminate object31 reflectors Il are used on the red lights, these reflectors being soplaced that the light is directed in one c'ase on the cathode alone andin the other case on the object alone. The tube of Figure 5 may behooked up as shown in Figure 7, the scanning and focusing coilsbeingomitted from the sketch in the interests of simplicity, it beingunderstood that they are to be used in the operation of the device.

Cathode 32 is connected with anode finger I2 in series with the anodesource 39. 'I'he usual collecting source 40 is connected to thecollecting plate IB, the output appearing across output resstor 4I inoutput leads 42 exactly asin the previous instance. Thus, lthere willfall upon the cathode surface 32'two different illuminations. One, a lowwavelength uniform illumination from the red lamps 46 which causes auniform emission of low velocity electrons from the cathode. The otherillumination is an optical image of light having a wavelength rangecontaining short wavelengths which will produce electrons from cathode32 falling into a different velocity catefA gory and having higheraverage velocities.

' Thus there will be-emitted from cathode 32 electrons falling into twovelocity categories. One category, made up of a uniform component of lowvelocity electrons, the other category, a

non-uniform electron image of higher velocity electrons. The grid,assuming that no optical image is thrown upon the cathode,will assume anegative charge because electrons reaching the grid collect upon theinsulating layer and leak on' to the base wire, thus reaching anequilibrium value at perhaps in the neighborhood` ofthreequarters of avolt. This will form a space charge back of the grid.

When this equilibrium is reached, that is, the equilibrium ldue to thebombardment by low velocity electrons, the charge on the grid will beuniform throughout. When, however, theoptical image reaches the cathodesurface, electrons of higher velocity are emitted, which, .due to theirhigher velocity can reach the grid and thus charge l the grid morenegatively at those .points wherethey do reach it and by an amount inproportion to the numbersreaching it. Ina'smuchas the number of highvelocity electrons reaching the grid at any elementary area thereof willbe dependent upon the illumination of the cathode on correspondingelementary areas thereof by the Y optical image, it can be seen that acharge pattern is built up upon the grid, this charge pattern modulatingthe electrons Apassing through the grid dueto the pun of the anode lz,thus :orm-

ing in the space between the grid and the anode I a new electron imageof higher intensity which In this instance, the grid becomes morenegative due to the.v action of the optical image emission, andmodulation is downward.`

Ihave found that by the building up of a space charge behind the gridand then'forming a charge pattern on the gridlin accordance with anoptical image pattern, that I have increased the overall sensitivity ofthe dissector tube over one thousand times, thus decreasing enormouslythe amount of amplification necessary in the, circuits utilized tovhandle and make useful train produced by scansion.

In the'embodimentshown in Figure 8, the source of the uniform electronstream is a lament Il and the lament is surrounded by a cylindricalelectrode 52 which tends to form a luni-potential space around thecathode emitter so that the electrons will be uniformly acceleratedtoward the grid i, in this case provided with a photoelectric mosaic andformed as shown in Figures 3 and 4 or other charge storage electrodes.such as are described and claimed in a companion applfca'tiomSerial No.30,118, led July 6, 1935 fo'r a Charge storage dissector.

In Figure iil the end wall 2 is left free from any.

iilm so that the 'optical image'is projected through ontothe chargestorage electrode 5, the filament assembly being so .small that noappreciable amount oi.' light is intercepted thereby. Electrons from thefilament charge the mosaic surlface 2O a uniform potential and theemission of electrons from the mosaic surface due to the ligh't from.the optical image' falling thereon causes the formationi'l f a chargeimage. This charge image modulates the electrons from the filament whichare l:being drawn through the apertures in the charge storage electrode5, and the modulated stream'is then accelerated toward the anodeaperture III selected areas thereof the signal passing through tobecollected by the collecting plate Il.

vThe space between the charge storage electrode or grid l and thecollecting anode is partially surrounded by a illmjil on the inside ofthe envelope which acts asl an electrostatic focusing aid and which isusually maintained at the,

` to a movement ofthe object occurs .during scansion, the pictureproduced toward the end of the scanningl cycle is 4simply anintermediate picture between that which was shown at the beginning ofthe scanning cycle` and that which is to come on the following scanningcycle. Even when objects move with the highest speeds customarilyoccurring withinthe perception of the human eye there is plenty of timefor the charge pattern

